School shootings, mass killings are 'contagious,' study finds

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Parents wait for news after a shooting at Marjory Stoneman Douglas High School in Parkland, Florida, on Wednesday, February 14. At least 17 people were killed at the school, Broward County Sheriff Scott Israel said. The suspect, 19-year-old former student Nikolas Cruz, is in custody, the sheriff said. The sheriff said he was expelled for unspecified disciplinary reasons.

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Investigators at the scene of a mass shooting at the First Baptist Church in Sutherland Springs, Texas, on Sunday, November 5, 2017. A man opened fire inside the small community church, killing at least 25 people and an unborn child. The gunman, 26-year-old Devin Patrick Kelley, was found dead in his vehicle. He was shot in the leg and torso by an armed citizen, and he had a self-inflicted gunshot to the head, authorities said.

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A couple huddles after shots rang out at a country music festival on the Las Vegas Strip on Sunday, October 1, 2017. At least 58 people were killed and almost 500 were injured when a gunman opened fire on the crowd. Police said the gunman, 64-year-old Stephen Paddock, fired from the Mandalay Bay hotel, several hundred feet southwest of the concert grounds. He was found dead in his hotel room, and authorities believe he killed himself and that he acted alone. It is the deadliest mass shooting in modern US history.

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Police direct family members away from the scene of a shooting at the Pulse nightclub in Orlando in June 2016. Omar Mateen, 29, opened fire inside the club, killing at least 49 people and injuring more than 50. Police fatally shot Mateen during an operation to free hostages that officials say he was holding at the club.

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In December 2015, two shooters killed 14 people and injured 21 at the Inland Regional Center in San Bernardino, California, where employees with the county health department were attending a holiday event. The shooters, Syed Rizwan Farook and his wife Tashfeen Malik, were later killed in a shootout with authorities. The pair were found to be radicalized extremists who planned the shootings as a terror attack, investigators said.

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Police search students outside Umpqua Community College after a deadly shooting at the school in Roseburg, Oregon, in October 2015. Nine people were killed and at least nine were injured, police said. The gunman, Chris Harper-Mercer, committed suicide after exchanging gunfire with officers, a sheriff said.

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A man kneels across the street from the historic Emanuel African Methodist Episcopal Church in Charleston, South Carolina, following a shooting in June 2015. Police say the suspect, Dylann Roof, opened fire inside the church, killing nine people. According to police, Roof confessed and told investigators he wanted to start a race war. He was eventually convicted of murder and hate crimes, and a jury recommended the death penalty.

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Police officers walk on a rooftop at the Washington Navy Yard after a shooting rampage in the nation's capital in September 2013. At least 12 people and suspect Aaron Alexis were killed, according to authorities.

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Connecticut State Police evacuate Sandy Hook Elementary School in Newtown, Connecticut, in December 2012. Adam Lanza opened fire in the school, killing 20 children and six adults before killing himself. Police said he also shot and killed his mother in her Newtown home.

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James Holmes pleaded not guilty by reason of insanity to a July 2012 shooting at a movie theater in Aurora, Colorado. Twelve people were killed and dozens were wounded when Holmes opened fire during the midnight premiere of "The Dark Knight Rises." He was sentenced to 12 life terms plus thousands of years in prison.

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A military jury convicted Army Maj. Nidal Hasan of 13 counts of premeditated murder for a November 2009 shooting rampage at Fort Hood, Texas. Thirteen people died and 32 were injured.

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Jiverly Wong shot and killed 13 people at the American Civic Association in Binghamton, New York, before turning the gun on himself in April 2009, police said. Four other people were injured at the immigration center shooting. Wong had been taking English classes at the center.

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Pallbearers carry a casket of one of Michael McLendon's 10 victims. McLendon shot and killed his mother in her Kingston, Alabama, home, before shooting his aunt, uncle, grandparents and five more people. He shot and killed himself in Samson, Alabama, in March 2009.

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Virginia Tech student Seung-Hui Cho went on a shooting spree on the school's campus in April 2007. Cho killed two people at the West Ambler Johnston dormitory and, after chaining the doors closed, killed another 30 at Norris Hall, home to the Engineering Science and Mechanics Department. He wounded an additional 17 people before killing himself.

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Mark Barton walked into two Atlanta trading firms and fired shots in July 1999, leaving nine dead and 13 wounded, police said. Hours later, police found Barton at a gas station in Acworth, Georgia, where he pulled a gun and killed himself. The day before, Barton had bludgeoned his wife and his two children in their Stockbridge, Georgia, apartment, police said.

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Eric Harris, left, and Dylan Klebold brought guns and bombs to Columbine High School in Littleton, Colorado, in April 1999. The students gunned down 13 and wounded 23 before killing themselves.

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In October 1991, George Hennard crashed his pickup through the plate-glass window of Luby's Cafeteria in Killeen, Texas, before shooting 23 people and committing suicide.

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James Huberty shot and killed 21 people, including children, at a McDonald's in San Ysidro, California, in July 1984. A police sharpshooter killed Huberty an hour after the rampage began.

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Prison guard George Banks is led through the Luzerne County courthouse in 1985. Banks killed 13 people, including five of his children, in Wilkes-Barre, Pennsylvania, in September 1982. He was sentenced to death in 1993 and received a stay of execution in 2004. His death sentence was overturned in 2010.

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Officers in Austin, Texas, carry victims across the University of Texas campus after Charles Joseph Whitman opened fire from the school's tower, killing 16 people and wounding 30 in 1966. Police officers shot and killed Whitman, who had killed his mother and wife earlier in the day.

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Howard Unruh, a World War II veteran, shot and killed 13 of his neighbors in Camden, New Jersey, in 1949. Unruh barricaded himself in his house after the shooting. Police overpowered him the next day. He was ruled criminally insane and committed to a state mental institution.

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Story highlights

A study finds that mass killings spread contagiously

The window for copycat killings lasts about 13 days

Experts say availability of handguns, media coverage is to blame for spike in U.S. mass killings

(CNN)Mass killings and school shootings spread "contagiously," a study found, where one killing or shooting increases the chances that others will occur within about two weeks.

The study, published in July the journal PLOS ONE, found evidence that school shootings and mass killings -- defined as four or more deaths -- spread "contagiously," and 20% to 30% of such killings appear to be the result of "infection." The contagion period lasts about 13 days, researchers found.

Researchers gathered records of school shootings and mass killings from several data sets and fit them into a mathematical "contagion model." The spread they found was not dependent on location, leading researchers to believe that national media coverage of a mass shooting might play a role. On average, mass shootings occur about once every two weeks in the United States and school shootings happen about once a month, the study said.

"What we believe may be happening is national news media attention is like a 'vector' that reaches people who are vulnerable," said Sherry Towers, a research professor at Arizona State University and lead author of the study.

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Those vulnerable people are those who have regular access to weapons and are perhaps mentally ill, Towers said. Once "infected" with knowledge of a shooting from national media coverage, data shows that a person is more likely to commit a similar crime.

"When at least three people are shot, but less than four people are killed, the media reports tended be local," Towers said. These shootings that received local news coverage, but no national news coverage, did not have the same contagious effect, according to Towers.

More tips from the public means a greater chance that tragic mass killings can be prevented, said Newman, who was not involved in the study.

"While there's a spike in shootings following an incident, there's an even bigger spike in reported plots," Newman said. "This is because people are vigilant and come forward with their suspicions and concerns."

Newman said the biggest hurdle to preventing school shootings is making it possible for people with information to report to authorities.

"If we want kids to come forward with information, we have to remind them these horrific crimes are happening," Newman said. "It should be part of a regular school curriculum to remind kids these things are going on."

Jack Levin, a criminologist at Northeastern University, said it's the amount of media coverage that matters.

"It's the excessive media attention that creates the copycat phenomenon. We make celebrities out of monsters," Levin said, noting that there are trading cards, action figures and magazine covers featuring murderers.

Researchers behind the new study also found that states with higher gun ownership were more likely to have mass killings and school shootings. On the contrary, states with tighter firearm laws had fewer mass shootings.

Levin said he believes a high number of handguns is partially responsible for the high rate of mass shootings in the United States.

"We have so many semi-automatic weapons that can be easily concealed, and taken from the home and used on classmates or whoever," he said. "The real problem in (the United States) has to do with handguns being in the hands of the wrong people. But you can't blame it all on guns. (The United States) leads the Western world in nongun homicides, too."

Towers knows firsthand the terror that shooting incidents can send through a community. She was traveling to a meeting at Purdue University in Indiana in January 2014 when the campus was locked down after reports of gunshots. Andrew Boldt, a 21-year-old Purdue senior, was fatally shot by another student, Cody Cousins. As details surrounding the shooting slowly emerged, Towers said she felt a mix of worry, relief, guilt -- and eventually, curiosity.

"It struck me as odd that other shootings occurred around the same time," Towers said. "I knew that day that I wanted to look into this further."

Collecting information about school shootings and mass killings wasn't easy, Towers said, noting "right now there is no federal database on these tragedies."

Towers said there are still a number of important questions left unanswered, and creating consistent data about these incidents is the first step.

"An official database needs to be compiled," Towers said. "The dynamic in the society needs to be addressed so we can fix this public health crisis."